I've got the same problem - Don't know whether to choose skis or board. I've tried kite skiing on sand, with 100cm skis and proper ski boots. What I found was that the natural stance for skiing is to lean forward, which is a slightly different from the stance you need when kite skiing. When attached to the kite, you need to lean back a bit, but the ski boots restrict you slightly and it become somewhat more tiring. There is more friction when sand skiing so maybe one needs to lean back more than on snow. I haven’t tried snow kiting with snow board though, but I would expect it to very similar to land boarding (I am not an expert on land boarding). A real big advantage with snow board over skis for this trip to Iceland in particular, is that snowboard boots weight MUCH less than ski boots – and in the unlikely event that you need to walk in your boots, snow board boots would also be the preferred option.
I don't have snow board so I'll probably rent one for the trip.